1. Field of the Invention
The present invention relates to a display device comprising a thin film transistor (hereinafter referred to as a xe2x80x9cTFTxe2x80x9d) for controlling a current to be supplied to a light emissive element.
2. Description of the Related Arts
In recent years, electroluminescence (hereinafter referred to as xe2x80x9cELxe2x80x9d) display devices using an EL element have been attracting attention as possible replacements for CRTs and LCDs.
In addition, an EL display device provided with a thin film transistor functioning as a switching element for driving the EL element has been researched and developed.
FIG. 1 is an equivalent circuit diagram of an EL display device according to a related art.
Referring to FIG. 1, an EL display panel includes, on an insulating substrate 10, a plurality of scan signal lines 81 connected to a vertical driver 80 for supplying a scan signal, and a plurality of data lines 91 receiving a data signal Sig on a data input line 92 when sampling transistors SP1, . . . , SPk, SPk+1, . . . , SPn are turned on in accordance with the timing of a sampling pulse output from a horizontal driver 90 for supplying the data signal. Near each intersection between these lines 81 and 91, a switching TFT 30 connected to these lines 81 and 91, an element driving TFT 40 connected to the switching TFT 30, and an organic EL element 60 for emitting light by receiving a current from an element driving power source line 100 in accordance with a voltage applied to a gate of the element driving TFT 40 are disposed.
A storage capacitor 70 is provided between the TFTs 30 and 40, having one electrode 71 connected to a source 11s of the TFT 30, and the other electrode 72 receiving a potential common to all display pixels 200.
The horizontal driver 90 receives a timing signal and the like, such as a horizontal start pulse STH, while the vertical driver 80 receives a timing signal and the like, such as a vertical start pulse.
The drivers 80 and 90 also receive driving voltages Vvdd and Hvdd, respectively, for driving these drivers. Each of the driving voltages Hvdd and Vvdd drives a shift register forming one of the drivers.
The sampling pulses are sequentially output from the horizontal driver 90 based on the start signal, and the sampling transistors SP are turned on in response to the sampling pulse, whereby a data signal Vdata1 on the data input line 92 is supplied to the data signal line 91. A gate signal is applied from the gate signal line 81 to a gate 13 of the first TFT 30, thereby turning on the first TFT 30. As a result, a drain signal is supplied to a source 11s of the TFT 30, and a voltage Vdata2 obtained at this point in time is applied to a gate 43 of the second TFT 40, thereby turning on the second TFT 40 and causing a current to flow from the element driving power source line 100 to the EL element 60 in accordance with a gate voltage Vdata2, such that light is emitted from the EL element 60.
A power source circuit 300 for generating the driving voltages Vvdd and Hvdd for driving the drivers 80 and 90, and an element driving power source Pvdd shown in FIG. 1 will next be described.
FIG. 2 is a block diagram of the power source circuit according to a related art.
Referring to FIG. 2, the power source circuit 300 includes a driver driving voltage generation circuit 320 for generating the driving voltages Hvdd and Vvdd for driving the drivers 80 and 90, and an element driving voltage generation circuit 330 for generating the element driving voltage Pvdd. The driving voltage generation circuits 320 and 330 are each formed by a DC/DC converter for converting a voltage of a power source 310, say, converting a voltage of 15V to a voltage of 12V.
The voltages of the driving voltage generation circuits 320 and 330 are provided to the drivers 80 and 90 and the element driving power source line 100.
In conventional EL display devices, however, when the display device is turned off after use, within the power source circuit 300, the flow of power is only suspended to the power source 310 and the circuits 320 and 330. When the voltage Vdata2 applied to the gate of the second TFT 40 falls before the element driving voltage Pvdd, a large amount of current may instantly flow to the EL element 60 from the power source line 100, even though control by the second TFT 40 is not possible. When this occurs, degradation of an emissive layer 66 of the organic EL element 60 may result.
The present invention has been conceived in view of the above-described problems, and an object thereof is to provide a display device capable of preventing degradation of an emissive layer as would otherwise be caused by excessive and instantaneous emission of light from an EL element when the display device is turned off.
According to one aspect of the present invention, a display device includes a vertical driver for supplying a scan signal to a plurality of gate signal lines; a horizontal driver for supplying a data signal to a plurality of data signal lines crossing said plurality of gate signal lines; a switching element connected to the gate signal line and the data signal line at an intersection between these signal lines; an emissive element connected to said switching element; an element driving power source line for supplying electric power to said emissive element; and a sequence circuit for stopping supply of electric power to said element driving power source line before stopping supply of a driver driving power source.
According to another aspect of the present invention, the switching element is composed of first and second thin film transistors, the first thin film transistor has a first region of an active layer connected to said data signal line, a gate connected to said scan signal line, and a second region of the active layer connected to a gate of the second thin film transistor, and the second thin film transistor has a first region of an active layer connected to said element driving power source line, and a second region of the active layer connected to one electrode of said emissive element.
According to a further aspect of the present invention, the sequence circuit includes first and second transistors; first and second resistors; and a third resistor connecting said first and second transistors, said first transistor has an emitter connected to said first resistor which is connected to an element driving power source generation circuit, a base connected to a power source, and a collector connected to ground, and said second transistor has an emitter connected to said second resistor which is connected to a driver driving power source generation circuit, a base connected to said third resistor, which is connected to the emitter of said first transistor, and to the other electrode of a capacitor whose one electrode is connected to ground, and a collector connected to ground.
According to a further aspect of the present invention, the emissive element is an electroluminescence element.
With the present invention, a display device capable of preventing degradation of an emissive element which would otherwise be caused by a large amount of current instantly flowing to the emissive element when the display device is turned off can be provided.